The role of the lymphatic system in the absorption and distribution of a variety of radiolabelled antitumor agents (CCNU, methyl-CCNU, guanazole, dibromomannitol, diethylstilbestrol, prednisolone, dexamethasone, 6-mercaptopurine, cytosine arabinoside, vincristine, cyclophosphamide, adriamycin, duanomycin, hexamethylmelamine, procarbazine, actinomycin D and o,p-DDD) in rats is under investigation. The relationship between extent of lymphatic absorption and the molecular weight and lipid solubility of the agents, their route of administration, and the vehicle or formulation in which they are given, is also being studied. Efforts are underway to develop a reproducible model system in rats for tumor metastasis via lymphatic channels, thus making it possible to evaluate the therapeutic advantage of treating such tumors with antitumor agents selectively taken up by lymphatics. The physiologic disposition of the plant-derived antitumor agent, maytansine, is under investigation in rats and in humans.